The present invention relates to a system and method for hydraulically controlling an automatic transmission for vehicles.
An automatic transmission for vehicles performs a shifting operation using hydraulic oil pressure. The hydraulic pressure is controlled by a hydraulic control system which generally includes an oil pump driven by an engine of the vehicle. In the hydraulic control system, a hydraulic oil pressure produced by the oil pump is regulated to a predetermined pressure, namely, a line pressure. A hydraulic oil amount discharged by the oil pump is controlled by a flow rate control valve so as to keep substantially constant when the engine speed is not less than a preset speed.
One example of the flow rate control valve includes a spool having an orifice and a drain hole (drain port), and a housing having a drain passage connected to an automatic transmission. The spool is displaceable in the housing in response to a hydraulic pressure difference between an upstream side of the orifice and a downstream side thereof. The amount of hydraulic oil passing through the orifice is kept constant by regulating the hydraulic oil amount discharged from the drain hole and the drain passage which are overlapped with each other during displacement of the spool. The spool is prevented by a stop from displacing beyond a position where the overlap between the drain hole and the drain passage is maximized. Japanese Patent Application First Publication No. 10-2415 discloses such a flow rate control valve useable in the hydraulic control system for automatic transmissions.